[ExecuTorch][WebGPU] Add aten.index.Tensor (1D-self gather)

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] aten.index.Tensor test suite (export + native golden) #20465
• -> [ExecuTorch][WebGPU] Add aten.index.Tensor (1D-self gather) #20464
• [ExecuTorch][WebGPU] Add clone op (aten.clone.default) #20463
• [ExecuTorch][WebGPU] Flatten landed-op test dirs to test/ops/test_<op>.py #20435
• [ExecuTorch][WebGPU] cat op test suite (cases.py op-test framework) #20399
• [ExecuTorch][WebGPU] Add cat op + ValueList graph support (aten.cat.default) #20398
• [ExecuTorch][WebGPU] permute_copy op test suite (cases.py op-test framework) #20397
• [ExecuTorch][WebGPU] Add permute_copy + IntList graph support (aten.permute_copy.default) #20396
• [ExecuTorch][WebGPU] slice_copy op test suite (cases.py op-test framework) #20395
• [ExecuTorch][WebGPU] Add slice_copy op (aten.slice_copy.Tensor) #20394
• [ExecuTorch][WebGPU] squeeze_copy + unsqueeze_copy test suites (cases.py op-test framework) #20393
• [ExecuTorch][WebGPU] Add squeeze_copy + unsqueeze_copy (flat copies) #20392
• [ExecuTorch][WebGPU] sigmoid op test suite (cases.py op-test framework) #20391
• [ExecuTorch][WebGPU] Add sigmoid op (aten.sigmoid.default) #20390
• [ExecuTorch][WebGPU] select_copy op test suite (cases.py op-test framework) #20363
• [ExecuTorch][WebGPU] Add select_copy op (aten.select_copy.int) #20362
• [ExecuTorch][WebGPU] view_copy op test suite (cases.py op-test framework) #20361
• [ExecuTorch][WebGPU] Add view_copy op (aten.view_copy.default) #20360
• [ExecuTorch][WebGPU] mul op test suite (cases.py op-test framework) #20359

Adds the WebGPU delegate handler for aten.index.Tensor, the 1D-self advanced-index
gather out[i] = self[index[i]] (output shape == index shape). This is the form the
VulkanPartitioner delegates -- it requires a 1D self and exactly one non-None index
(op_registry.py); 2D mask/freqs gathers stay on CPU. It mirrors the Vulkan delegate's
index_tensor op (IndexTensor.cpp + index_tensor_buffer.glsl) as a single compute
dispatch over the output elements, each reading the int32 index and gathering the
corresponding fp32 self element.

The op is composed as:

• index.wgsl: one workgroup-strided pass, out[i] = self[u32(index[i])], guarded by a
  numel bound; buffer-only, fp32 self/out, int32 index, 1D dispatch via the shared
  WebGPUUtils helpers (clamp workgroup size + 1D count).
• Index.cpp: validates the args (self/out tensors; indices ValueList with exactly one
  index tensor; fp32 self/out; int32 index; out numel == index numel), failing loud on
  any violation, then records the dispatch. row_width is dropped (always 1 for 1D self).

Differential Revision: D109478967

[pytorch-bot]

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• [ROCm] MI350 CI jobs will have longer queue times due to CI migration

❌ 5 New Failures, 2 Pending, 4 Unrelated Failures

As of commit 8ca983e with merge base 1b726b2 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224
• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 528002d9225067a494eaf5b217564ec79f5ada74b9f2441db951a2f12c1bb962 /exec failed with exit code 1
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t ffb443d966be4c04ee4967ccbac8dfd7e343d8f9e9c6ae3d38e11b0e7d10ee6c /exec failed with exit code 139
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t db4be7f130342a64e66592a21c64af63fd47a666c54d5b4fc2e731f514007099 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 25121a779c46c001b03d339dba65fc21fac50ed3ef5dad2891db2f173464fa4e /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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